A battery, also known as a galvanic or voltaic cell, uses a chemical oxidation-reduction reaction to produce electric current for powering a load in an electric circuit. FIG. 1 is a diagram of a generic lead-acid cell 100, which includes two electrodes 102, each with one end dipped in an electrolytic fluid 104, typically sulfuric acid, and each with the other end connected by a wire 106 to an external electric circuit 108. Each electrode 102 separately undergoes one-half of an electrochemical oxidation-reduction reaction to either produce or consume free electric charge. A lead anode 110, or negative electrode, is oxidized in a reaction that supplies electrons 112. A lead oxide cathode 114, or positive electrode, is reduced in a reaction that consumes electrons. A main requirement is that electrodes 102 be kept separate from each other so that electron transfer is forced to occur through wire 106 in external electric circuit 108. A separator 116, or porous partition, is therefore used to divide cell 100 into a left compartment 118a and a right compartment 118b. Separator 116 prevents electrodes 102 from coming into physical contact with each other and short-circuiting cell 100. Separator 116 permits electrolyte 104 to reside in the pores of the separator material and thereby facilitates diffusion of ions 120 between left compartment 118a and right compartment 118b. If separator 116 is insufficiently porous, ionic current flow through electrolyte 104 is hindered and thereby causes a charge imbalance that impedes, and may ultimately arrest, the electrochemical reaction.
Battery separators 116 are complex multi-component membranes that dictate the mechanical and electrical performance of the battery. The “recombinant cell” and the “flooded cell” are two commercially available lead-acid battery designs that incorporate different types of separators. One type of recombinant cell, a valve regulated lead acid (VRLA) battery, typically includes an absorptive glass mat (AGM) separator composed of microglass fibers. While AGM separators provide excellent porosity (>90%), low electrical resistance, and uniform electrolyte distribution, they are relatively expensive and fail to offer precise control over the recombination process or the rate of oxygen transport within the electrolyte. Furthermore, AGM separators exhibit low puncture resistance, causing more frequent short circuits. Manufacturing costs for the fragile AGM sheets are high. In some cases, battery manufacturers select thicker, more expensive separators to improve puncture resistance, even though electrical resistance increases with thickness.
In a flooded cell battery, only a small portion of the electrolyte is absorbed into the separator. Materials for flooded cell battery separators typically include porous derivatives of cellulose, polyvinyl chloride (PVC), organic rubber, and polyolefins. Microporous polyethylene battery separators are commonly used because of their ultrafine pore size, which inhibits “dendritic” growth of metallic deposits (a short circuit risk), while providing low electrical resistance, and exhibits high puncture strength, good oxidation resistance, and excellent flexibility. Such properties facilitate sealing the battery separator into a pocket or envelope configuration into which a positive or negative electrode can be inserted. A main drawback of current commercial polyethylene separators is that their porosities are much lower than the porosities of AGM separators, generally ranging from 50%-60%.
The term “polyethylene separator” is something of a misnomer because microporous separators must contain large amounts of a siliceous filler such as precipitated silica to be sufficiently acid-wettable. The volume fraction of precipitated silica and its distribution in the separator generally control its electrical properties, while the volume fraction of polyethylene, more particularly ultrahigh molecular weight polyethylene (UHMWPE), and its degree of orientation in the separator generally control its mechanical properties. Precipitated silica is hydrophilic and, because of its high surface area and the presence of surface silanol groups, precipitated silica easily increases the acid wettability of the separator web and thereby lowers the electrical resistivity of the separator. In the absence of silica, sulfuric acid alone would not wet the hydrophobic web and therefore ion transport would be prevented, resulting in an inoperative battery. The silica dispersed wettability component of the separator typically accounts for between about 55% and about 80% by weight of the separator, i.e., the separator has a silica-to-polyethylene (PE) weight ratio of between about 2:1 and about 3.5:1.
During the manufacture of polyethylene battery separators, precipitated silica is typically combined with UHMWPE, a process oil, and various minor ingredients to form a separator mixture. The separator mixture is extruded at an elevated temperature (up to 250° C.) through a sheet die to form an oil-filled sheet of a designated thickness and profile, before extraction of most of the process oil. The sheet is then dried to form a microporous polyethylene separator, and then slit into an appropriate width for a specific battery design. The polyethylene separator is delivered in roll form to lead-acid battery manufacturers where the separator is fashioned into “envelopes.” An electrode can then be inserted into a separator envelope to form an electrode package. Electrode packages are stacked so that the separator acts as a physical spacer and as an electrical insulator between positive and negative electrodes. The primary functions of the polyethylene contained in the separator are to provide mechanical integrity to the separator, so that it can be enveloped at high speeds, and to prevent grid wire puncture during battery assembly or operation. The polyethylene preferably has sufficient molecular chain entanglement to form a microporous web with high puncture resistance. An electrolyte is then introduced into the assembled battery to facilitate ionic conduction within the battery. Table 1 summarizes the functions of the battery separator components described above.
TABLE 1Battery separator components and their functionsComponentFunctionPolyethyleneMechanical propertiesSilicaWettability and porosityResidual OilOxidation resistanceAntioxidantOxidation resistanceVoids/PoresIon conduction
In response to the increased price of lead, battery manufacturers desire a separator with exceptionally low electrical impedence to achieve the same battery discharge rate with less active material (especially lead and lead oxide) in the electrodes. Some polyethylene separator manufacturers have used surfactants to promote separator wettability and lower electrical resistance through better wetting of available pores. However, surfactant is known to degrade lead-acid battery performance, and surfactants can migrate or they can decompose in a lead-acid battery environment. In an alternative approach, separator manufacturers have increased the percentage of process oil in their formulation in an attempt to increase the porosity of the finished separator, following extraction. However, increased oil content can cause more shrinkage during manufacturing so that the fixed rib pattern, which is imparted to the separator material during extrusion, cannot be maintained. In a third approach, subjecting the separator to treatment with a high-electric potential coronal discharge can be used to improve the wettability of the separator, but this effect is temporary. In a fourth approach, polymers containing functional groups that enhance wettability (e.g., ethylene-vinyl alcohol copolymers) can partly replace the hydrophobic UHMWPE polymer matrix of the separator; however, this can significantly reduce puncture resistance. It is therefore desirable to produce, with a cost-effective process, a microporous polyethylene separator having a material composition that provides good puncture resistance and high oxidation resistance while achieving very low electrical resistance.